A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing (i.e. pattern application) can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print (i.e. apply) the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed (i.e. applied) feature. It follows from equation (1) that reduction of the minimum printable (i.e. applicable) size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable (i.e. applicable) feature size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, or for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma (LPP) sources, discharge plasma (DPP) sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
In a lithographic apparatus, a radiation beam used to apply patterns to a substrate will traverse (e.g. reflect off, pass through, be refracted by, and the like) one or more optical elements (e.g. lenses or mirrors). Such optical elements may be present in an illumination system of the lithographic apparatus in order to condition the radiation beam. Alternatively or additionally, such optical elements may be present in the projection system of the lithographic apparatus that is used to project the radiation beam onto a substrate. These optical elements will not be perfectly transmissive or reflective with respect to the radiation beam, and this will result in heating of the optical elements as the radiation beam traverses them. This is particularly true of EUV lithographic apparatus at the present time. This is because EUV lithographic apparatus employ mirrors as the optical elements of the illumination system and projection system, and these mirrors have a relatively low reflectivity resulting in correspondingly high heat absorption. The same or similar problems may also be encountered in other forms of lithographic apparatus that use radiation other than EUV radiation.
Heat absorption by an optical element can cause deformation of the optical element. Such deformation is more likely to occur, or is likely to be more pronounced or exaggerated, when the radiation beam is not uniformly distributed across the optical element, such as for example when the radiation beam forms a quadruple or dipole illumination mode. This is because the heating, and thus the deformation, may occur more locally in the region of the poles forming the illumination mode. Deformation of the optical element can be detrimental to the optical performance of the optical element which may, in turn, lead to a poor performance of the lithographic apparatus as a whole (for example, in terms of the accuracy or consistency with which patterns can be applied to a substrate using that lithographic apparatus).
Correction (i.e. at least partial reduction) of the deformation of the optical elements may be undertaken by mechanically deforming the optical element, or by heating of the optical element. Mechanical correction is limited in terms of the number of locations of or on the optical element where the deformation can be applied, and becomes more complex and costly if such correction by mechanical deformation is desired at a large number of locations (e.g. a number of locations around the optical element, or for a number of locations at different optical elements).